Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flowmeter includes one or more conduits (also called flow tubes) that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by a driver, e.g., an electromechanical device, such as a voice coil-type actuator, that perturbs the conduit in a periodic fashion. Mass flow rate may be determined by measuring time delay or phase differences between motions at the transducer locations. Two or more such transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduits, and are typically located at positions upstream and downstream of the driver. Instrumentation receives signals from the pickoff sensors and processes the signals in order to derive a mass flow rate measurement.
Flowmeters may be used to perform mass flow rate measurements for a wide variety of fluid flows. One area in which Coriolis flowmeters can potentially be used is in the metering of oil and gas wells. The product of such wells can comprise a multiphase flow, including the oil or gas, but also including other components, including water and air, for example, and/or solids. It is, of course, highly desirable that the resulting metering be as accurate as possible, even for such multiphase flows.
Coriolis meters offer high accuracy for single phase flows. However, when a Coriolis flowmeter is used to measure multiphase fluids such as fluids including entrained gas, the accuracy of the meter can be significantly degraded. This is similarly true for flows having entrained solids and for mixed-phase fluid flows, such as when hydrocarbon fluids contain water.
Entrained gas is commonly present as bubbles in the flow material. The size of the bubbles can vary, depending on the amount of air present, the pressure of the flow material, and the temperature. A related and significant source of error arises from fluid decoupling. Fluid decoupling results from the motion of the gas bubbles with respect to the liquid as a result of the vibration of the tube. The relative motion of the gas bubbles with respect to the liquid is driven by a buoyant force that is similar to the force that causes bubbles to rise to the surface under the influence of gravity. However, in a vibrating tube, it is the acceleration of the vibrating tube that causes the bubbles to move more than the acceleration of gravity. Because the dense fluid has more mass than the light bubbles, the bubbles have greater acceleration than the fluid in the direction of the tube acceleration. Due to the greater acceleration of the bubbles, on each oscillation of the flow conduit, the bubbles move further than the flow conduit. Additionally, the bubble motion causes some of the fluid to move less than the flow conduit. This is the basis of the decoupling problem. As a result, the fluid that has the lower vibrational amplitude undergoes less Coriolis acceleration and imparts less Coriolis force on the flow conduit than it would in the absence of bubbles. This results in the flow rate and density characteristics being under-reported (negative flow and density errors) when entrained gas is present. Compensating for fluid decoupling has been difficult because there are several factors that determine how much the bubbles move with respect to the fluid. Fluid viscosity is an obvious factor. In a very viscous fluid, bubbles (or particles) are effectively frozen in place in the fluid and little flow error results. Another influence on bubble mobility is the bubble size. The drag on a bubble is proportional to the surface area, whereas the buoyant force is proportional to the volume. Therefore, very small bubbles have a high drag to buoyancy ratio and tend to move with the surrounding fluid. Small bubbles subsequently cause small errors. Conversely, large bubbles tend not to move with the surrounding fluid and result in large errors. The same holds true for particles. Small particles tend to move with the fluid and cause small errors.
The density difference between the fluid and the gas is another factor that may contribute to flowmeter inaccuracy. The buoyant force is proportional to the difference in density between the fluid and the gas. A high pressure gas can have a high enough density to affect the buoyant force and reduce the decoupling effect.
In addition to measurement errors, the effect of multi-phase flow on Coriolis meters is increased by damping on the flow conduit, resulting in the diminishment of flow conduit vibratory amplitude. Typically, meter electronics compensate for this diminished amplitude by increasing the drive energy, or drive gain, in order to restore the amplitude. Even very small amounts of gas can cause a large increase in drive gain.
To correct for errors due to multi-phase flow, measured variables including density, mass flow, and volume flow are used from a period of single phase flow (liquid only)—these values are referred to as hold values. Hold values are used during multi-phase flow to replace or improve the accuracy of measured variables. Currently, hold values are determined at a user specified point in time before multiphase conditions exist.
Previously, drive gain has been used to determine whether or not there is multi-phase flow in the meter. If a meter's drive gain goes above a certain threshold, then the fluid in the meter is considered to be multi-phase flow and corrective action can be taken to improve the accuracy of the measured values. In prior art meters, a default value for drive gain threshold is used. In practice, the default value must be set conservatively high so that it will work for most applications. This must be done for three reasons: (1) Every Coriolis meter has a different base drive gain. This is the drive gain required to drive the flow conduit under purely single phase flows. Because of this, the drive gain must be high enough to work for every meter. For example, a typical nominal drive gain for one meter family might be 2%, whereas the nominal value for another meter family might be 20%. This nominal value depends on many things, including magnet strength and design, coil design, and meter size/stiffness; (2) Purely liquid multi-component mixtures composed of one or more different density liquids will have the same decoupling effect as gas and liquid fluids, although much smaller. Errors are mostly negligible in purely liquid multi-component flow, but there can still be small increases in drive gain that should not be treated as gas. Again, the threshold must be high enough to not mistake purely liquid flow as gas and liquid flows; and (3) For some applications, there may never be periods of pure liquid from which to base hold values. However, often times there are periods of mostly liquid where only small traces of gas may exist. The drive gain threshold is set high enough so that these periods are treated as pure liquid so that hold values may be created, and the periods of very high gas may still be corrected. The default value works for some applications. However, for applications where there may be only small amounts of gas entering the meter, the default threshold may be too high. Due to drive gain's sporadic nature, and the potential that drive gain threshold is set too high, this method does not always produce hold values from periods of minimal or no gas. For applications where there is always enough gas such that the drive gain never drops below the threshold, the default threshold is too low.
In cases where the default threshold isn't suitable, an operator must manually configure the flowmeter to use a more accurate value. This process requires the operator to collect and monitor data from the meter and manually set a new threshold value. Should process conditions change over time, this threshold may need to be readjusted. This is a timely and expensive process. Aside from the time wasted, there are also safety regulations which sometimes prevent the convenient use of a laptop to connect to meters in the field.
There remains a need in the art for a vibratory flowmeter that mitigates problems associated with setting an appropriate drive gain threshold for dealing with multi-phase flow. Embodiments herein provide methods used to determine an ideal threshold. Further, these embodiments disclose how often to output data, and how frequently to try to find related data values, such as hold values, for example.